The present invention relates to instruments for measuring atmospheric humidity.
In particular, the present invention relates to a capacitive humidity transducer.
It is well recognized that for many purposes it is desirable to be able to give an indication of the humidity in the atmosphere. For most purposes the best way to measure the humidity is by way of an electrical structure.
In this latter connection it is known to take two different approaches. One of these approaches involves providing a circuit the resistance of which will vary with variations in humidity. This approach is not entirely satisfactory for all purposes because of inaccuracies which are inherent in a resistive type of humidity measurement. For example, the resistive elements which respond to changes in humidity easily become contaminated so as to introduce undesirable inaccuracies in such structures.
A capacitive humidity transducer is therefore preferable for most purposes, but at the present time the known capacitive humidity transducers suffer from serious drawbacks. Conventional capacitive humidity transducers include a core in the form of a film which is active with respect to water absorption and which has a dielectric constant which varies according to the extent of water absorbed by such a film. However, the only known constructions which up to the present time utilize this type of capacitive humidity transducer support the film at its peripheral region while leaving the part of the film which is surrounded by the supporting structure completely free and unsupported. Such a construction is resorted to so that it becomes possible to provide at the opposed faces of the film which is supported only at its periphery conductive coverings acting as plates of a condenser, and these conductive coverings are then respectively connected into a suitable electrical circuit to provide a capacitance which can be measured. A primary problem encountered with such a construction resides in the fact that the unsupported dielectric film structure expands and contracts during changes in water absorption and readily assumes a wavy rather than a planar construction and is undesirably influenced by passing currents of air and the like. Moreover, in order to provide electrically conductive coverings at the opposed faces of such a film, a material which may be the same as that of the film is applied to both faces thereof with this material having in its interior electrically conductive particles of graphite or other metals which engage each other to provide for the desired electrical conductivity at the opposed surfaces of the film core. The result is that the active film structure extends not only from the intermediate layer between the conductive particles but also into the spaces between these particles, so that it is not possible with such a construction to provide a precisely determined film thickness.
Furthermore, because of the manner in which it is necessary with the known constructions to support the dielectric film, the film of necessity must have a thickness which is greater than an ideal thickness and at the same time not only this latter factor of the undesirably large thickness of the film but also the use of particles interspersed in a plastic material to achieve electrical conductivity result in a structure which is undesirably sluggish in its response. In other words with such known constructions there is an undesirably long interval between the time when atmospheric humidity changes and the time when the change in the humidity is indicated by the known transducers.